1. Field of the Invention
The present invention generally relates to semiconductor devices and, more particularly, to a semiconductor device which is formed by encapsulating a semiconductor element mounted on a lead frame by a seal resin.
2. Description of the Related Art
FIG. 1 is a cross-sectional view of a semiconductor device using a conventional lead frame. The semiconductor device 1 shown in FIG. 1 has a so-called lead type package, which is provided with many lead terminals extending from a resin seal part which encapsulates a semiconductor chip.
In FIG. 1, a semiconductor chip 2 is fixed on a die stage 4 of a lead frame 3 by a die-bonding material 5. Electrodes of the semiconductor chip 2 are connected to inner leads 7 by bonding wires 6, and the semiconductor chip 2, the die stage 4, the bonding wires 6 and the inner leads 7 are encapsulated by a seal resin 8. From the seal resin 8, outer leads 9 extend as terminals for external connection.
Generally, a lead frame is formed by processing and patterning a copper alloy plate by stamping, etching, etc. After patterning the copper alloy plate, silver (Ag) plating is applied at the end of each inner lead so as to facilitate wire bonding. If necessary, an organic discoloration preventing agent is applied to the whole lead frame. Zinc (Zn), lead (Pb), chromium (Cr), etc. are contained as additives in the copper (Cu) alloy forming the lead frame.
In a manufacturing process of the above-mentioned semiconductor device 1, the surface of the lead frame 3 is oxidized thermally in a heating process such as a wire-bonding process. That is, heating of the lead frame 3 forms a thin film of copper oxide on the surface. In such a case, a phenomenon occurs in which a very small amount of additive elements in the copper alloy is separated and condensed near a boundary between a copper oxide layer and a base material. Such a part into which the additive elements are condensed has a comparatively brittle characteristic. That is, thermal oxidation of the base material of the copper alloy forms a brittle layer between the copper oxide layer on the surface and the inner base material.
Additionally, the discoloration-preventing agent applied to the surface of the lead frame 3 also forms a brittle layer when a thermal oxidation film is formed in a heating process such as a wire-bonding process. That is, a part of elements contained in the discoloration preventing agent may form a brittle layer near the boundary between the copper base and the copper oxide film.
If a semiconductor device is heated in a solder reflow process in a case in which the semiconductor device, which is formed by encapsulating the lead frame 3 in which the above-mentioned brittle layer is formed, is mounted onto a mounting substrate, etc., a crack may occur in the brittle layer. If a moisture in the seal resin turns into steam and enters such a crack, as shown in FIG. 2, exfoliation may occur between the lead frame 3 (a copper alloy which is the material of the die stage 4) and the seal resin 8 (copper oxide layer of the surface of the die stage 4), and there may occur a problem of package cracking or internal cracking. Such a problem tends to appear more notably, if a mounting temperature rises due to use of a lead-free solder.
Additionally, an effect similar to the effect of the present invention may be obtained in a blackening treatment which forms needle crystals of cupric oxide (CuO) on a surface of a copper alloy process. However, depending on the state of formation of copper dioxide, exfoliation may occur between the copper oxide and the base material of the copper alloy when the completed semiconductor device is subjected to a solder reflow process, and there is no way to distinguish them by their appearance.